Broadband Acoustic Environment at a Tidal Energy Site in Puget Sound

Admiralty Inlet, Puget Sound, Washington, has been selected as a potential tidal energy site. It is located near shipping lanes, is possibly a highly variable acoustic environment, and is frequented by the endangered Southern Resident killer whale (SRKW). Resolving environmental impacts is the first step to receiving approval to deploy tidal turbines. Several monitoring technologies are being considered to determine the presence of SRKW near the turbines. Broadband noise level measurements are critical for determining design and operational specifications of these technologies. Acoustic environment data at the proposed site were acquired at different depths using a cabled vertical line array from two cruises during flood and ebb tidal periods in May and June 2011. The ambient noise level decreases approximately 5 dB re 1 mu Pa per octave for frequency ranges of 1-70 kHz and increases approximately 5 dB re 1 mu Pa per octave for the frequency from 70 to 100 kHz. The difference between noise pressure levels in different months varies from 10 to 30 dB re 1 mu Pa for the frequency range below 70 kHz. Commercial shipping and ferry vessel traffic were found to be the most significant contributors to sound pressure levels for the frequency range of 1-50 kHz, and the variation could be as high as 30 dB re 1 mu Pa. These noise level measurements provide the basic information for designing and evaluating both active and passive monitoring systems proposed for deployment and operation of a tidal power generation alert system.